Nanoelectronic devices based on nanomaterials such as nanowires, carbon nanotubes, graphene and transition metal dichalcogenides offer extremely large surface-to-volume ratios, high carrier mobility low power consumption and high compatibility for the integration with modern electronic technologies. These distinct advantages are being explored for a variety of sensing applications in both liquid and gas phases. In particular, chemical vapour sensing is uniquely positioned to elucidate the fundamental molecule-nanomaterial interaction and provide a test bed for evaluating nanoelectronic-sensing performance without interference from solvent background typically seen in liquid-based detection. The current signal of a nanoelectronic sensor can in general be expressed as:I=(G+{tilde over (G)})·(V+{tilde over (V)})=G·V+G·{tilde over (V)}+{tilde over (G)}·V+{tilde over (G)}·{tilde over (V)},  (1)where G is the conductance, determined by the charge density of the device, Q; {tilde over (G)} is the conductance fluctuation and is related to the modulation of the charge density, {tilde over (Q)}; V is the direct current (DC) voltage and {tilde over (V)} is the alternating current (AC) excitation. Exploration of different terms in equation (1) can lead to different sensing mechanisms, for example, the first three terms have been employed in DC sensing, impedance sensing and noise sensing, respectively. In contrast, the fourth term explores the heterodyne mixing signal between conductance modulation and AC excitation, and has unfortunately been ignored in electronic-sensing techniques owing mainly to the lack of gain in conventional two-terminal devices. However, as reported in this disclosure, utilizing the heterodyne mixing current as the sensing signal in a high-frequency graphene mixer will not only open up a new possibility of probing the fundamental molecule-graphene interaction, but, surprisingly, enable a rapid and sensitive nanoelectronic vapour sensor that significantly outperforms the current state-of-the-art.
Presently the most common sensing mechanisms for nanoelectronic sensors, such as chemiresistors and transistor-based sensors rely on the detection of charges. Charge transfer between the adsorbed molecules land the nanomaterial changes the surface charge density, thus altering the Fermi energy and conductance of the sensors. Sensing is achieved by monitoring the DC conductance change (first term in equation 1) as a result of molecule-sensor interaction. To date, semiconductor nanowires, carbon nanotubes, graphene and MoS2 have been explored as DC nanoelectronic vapour sensors is their extremely slow sensing response and recovery, typically on the order of tens to hundreds of seconds. AC impedance-sensing technique utilizing the second term in equation 1 has also been demonstrated in chemicapacitors. A carbon nanotube network-based chemicapacitor exhibited a detection limit of 50 ppb for dimethylmethylphosphonate (DMMP). However, a large device footprint (millimeter scale) is necessary for accurate capacitance measurement, and the use of chemoselective polymers in those devices significantly slows down the response time to hundreds of seconds. More recently, the low frequency noise spectrum of a graphene transistor was also used for chemical vapour sensing by exploiting the third term in equation 1. Selective gas sensing was achieved on a single pristine graphene transistor, but the device suffered severely from extremely poor sensitivity and slow response time (>100 s).
Unfortunately, the slow response for all existing nanoelectronic vapour sensors arises intrinsically from slow dynamics of interface-trapped charges and slow defect-mediated charge-transfer processes and therefore is difficult, if not impossible, to overcome within the current framework of available sensing mechanisms. As a result, device regeneration is achieved only through prolonged heating, current stimulation or ultraviolet radiation. Recently, various chemoselective surface coatings have been used to reduce the response and recovery time to only a few seconds. However, those coatings function only for a narrow set of vapour molecules and may possibly result in even slower response to other vapour molecules. All these drawbacks not only preclude studying the rapid dynamics of molecule-nanomaterial interaction, but also significantly hinder the employment of nanoelectronic sensors in applications like gas chromatography (GC), which require detection capability for a broad range of vapour analytes with sub-second response time and ppb-level sensitivity.